"If we elect Marco Rubio, every day will be a great day in America," Haley said, riffing on her signature phrase.

"I wanted somebody with fight, I wanted somebody with passion, I wanted somebody who had the conviction to do the right thing, but I wanted somebody humble enough to remember he works for all the people," Haley said.

"I wanted somebody who could show my parents that the best decision they ever made for their children was coming to America," said Haley, who is a first-generation Indian-American.

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She'll be on the campaign trail with Rubio through Saturday's primary.

The popular second-term governor's endorsement could help Rubio, who is facing attacks from both Texas Sen. Ted Cruz and former Florida Gov. Jeb Bush ahead of Saturday's vote.

The current leader in South Carolina, according to a CNN/ORC poll released Tuesday, is Donald Trump, with 38% support. He's trailed by Cruz with 22%, Rubio with 14%, Bush with 10%, Ben Carson at 6% and Ohio Gov. John Kasich at 4%.

Haley is just the latest South Carolina Republican whose support Rubio has picked up. He is also backed by Sen. Tim Scott and Rep. Trey Gowdy, two high-profile members of the state's congressional delegation.

Rubio sees Haley's endorsement as providing "a lift to come in second" in the Palmetto State's primary, a source close to the campaign told CNN.

It's also a sign, the source said, that "the disaster at the debate in New Hampshire is over" and Rubio now has firm standing over Bush.

Bush himself had told NBC News on Tuesday that Haley's endorsement would be critical.

"She is the probably the most meaningful endorsement," Bush said, adding that her support would be "powerful" and if he didn't get it, "it sends a signal that I got to work harder."

Speaking to reporters after a town hall event in Summerville, South Carolina, Bush said he was "disappointed" to not win Haley's support.

"She's a very good governor and should I win the nomination, there will be a role for her in the campaign, trust me. She's a great person," Bush said.

A source close to the Bush campaign said they had lobbied hard for Haley's support and hoped she would go with them, and the source acknowledged that this would make South Carolina even more difficult for Bush to win.

Haley has long made her distaste for Trump clear, including a shot at the Republican front-runner in the State of the Union response she delivered for the GOP this year.

"During anxious times, it can be tempting to follow the siren call of the angriest voices," Haley said during that speech from the governor's residence in Columbia. "We must resist that temptation. No one who is willing to work hard, abide by our laws, and love our traditions should ever feel unwelcome in this country."

Later Wednesday, Dan Scavino, a senior adviser to Trump, tweeted an article from conservative news website WND.com entitled "Refugee-lovin' governor sued for importing Muslims" about a lawsuit filed by an activist that seeks to prevent efforts to resettle refugees from Syria and Iraq in South Carolina. The lawsuit names Haley, the state Department of Social Services and two nonprofit agencies as defendants, GreenvilleOnline.com reported.